Benthic barrier mat and method

ABSTRACT

A benthic barrier in which a polygonal shaped mat having straight sides is mounted on a similarly polygonal frame, but in which the frame sides bow out slightly between corners, causing the mat to stretch slightly and become taut. The frame comprises a plurality of straight leg members and a number of corner members as required by said polygonal shape. The corner members encompass interior angles which are from about 1 to about 5 degrees greater than the interior angle of the polygon defined by the straight sides of the mat. The frame member legs are rigid, but slightly flexible such that when assembled into the mat as an assembled frame, the straight legs are forced to bow out slightly between the corners, pulling the mat taut.

FIELD AND BACKGROUND

The present invention relates to the field of “benthic barriers,” socalled because they are barriers to the bottom of an area of water. Thebenthic zone is the ecological region at the lowest level of a body ofwater such as a lake, including the sediment surface and somesub-surface layers. Such barriers are typically placed in “photiczones,” which are areas where enough sunlight reaches the benthic areato permit photosynthesis, allowing aquatic plant growth. BenthicBarriers are intended to block sunlight from reaching lake bottoms,thereby preventing photosynthesis and eliminating unwanted, aquaticplant growth in specific areas where benthic barriers are placed.

Know by common names such as, “lake weed control mats,” “seaweed mats”and simply, “weed mats,” crude versions of benthic barriers have beenused in the United States for well over 100 years. Early benthicbarriers were first fashioned of materials such as burlap and canvas toblock sunlight and were held on the bottom primarily with rocks.Primitive frames were sometimes constructed with iron bars car fencingto, hold the fabrics in place. In some mats, metal bars, likereinforcing rod, have been placed transversely across the mat. Othersattach frames on the perimeter of the fabric. Remarkably, many homemadeframes were made of wood (which floats). Other frames have beenfashioned from steel reinforcing rod, which is a soft, flexible bar,with a very low “bend moment.”

Some of these barriers today comprise a polymeric fabric mat. In some,the polymeric fabric mat is surrounded by a rectangular frame to whichthe mat is secured. Some mats include small gas relief openings in thebody of the mat, so that gas coming up from beneath the mat can escapethrough those openings. A mat sold by Lacey Inc. is provided withsemi-circular slits in the mat, to define openings with adjacent flaps.This allows the mats to settle to the bottom of the lake more quickly,and also facilitates the escape of gas which tends to bubble up beneaththe mat and lift the flaps out of the way.

SUMMARY OF THE INVENTION

The present invention is a benthic barrier in which a polygonal shapedmat having straight sides is mounted on a similarly polygonal frame, butin which the frame sides bow out slightly between corners, causing themat to stretch slightly and become taut. The frame comprises a pluralityof straight leg members and a number of corner members as required bysaid polygonal shape. The corner members encompass interior angles whichare from about 1 to about 5 degrees greater than the interior angle ofthe polygon defined by the straight sides of the mat. The frame memberlegs are rigid, but slightly flexible such that when assembled into themat as an assembled frame, the straight legs, are forced to bow outslightly between the corners, pulling the mat taut.

Surprisingly, this facilitates the removal of naturally occurring gasesfrom below the barrier, rather than collecting beneath the mat andmaking it unstable. These and other objects, advantages and features ofthe invention will be further understood and appreciated by reference tothe drawings and written description set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a preferred embodiment benthic barrier with matand frame assembled together, and with a support grid secured thereto onthe underside of the mat;

FIG. 2 is an enlarged view of a corner portion of the benthic barrier;

FIG. 3 is a perspective view of the mat, the assembled frame, and thesupporting grid exploded away from one another.

FIG. 4 is a plan view of the mat and frame members separate from eachother;

FIG. 5 is a plan view of a frame corner member;

FIG. 6 is a perspective view of frame members, which are positionedwithin receiving pockets of the mat, being assembled together;

FIG. 7A is a close up view of the joining ends of frame members spacedslightly apart; and

FIG. 7B is a close op view of the joining ends of frame members joinedtogether.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The benthic barrier 1 of the preferred embodiment comprises mat 10,frame 20, and optionally a grid under support 30, shown assembled inFIG. 1, and exploded away from one another in FIG. 3 (FIGS. 1 and 3).Frame 20 is formed of plural frame members 21 and 22 which comprise legswhich are straight when frame 20 is unassembled (FIG. 4), but which areforced to bow out slightly by the configuration of the corners 21 c whenassembled (FIGS. 1, 2 and 3). The barriers 1 can come in varyingpolygonal shapes and dimensions, such as rectangles 8 ft. by 8 ft., 12ft. by 14 ft., 12 ft. by 24 ft., etc. The various components of barrier1 are numbered herein as follows:

Benthic barrier  1 Mat 10 Gas relief ports 11 Corner cutaway 12 Framepockets 13 Frame 20 Corner frame members 21 Straight legs 21a and 21bCorner  21c Straight frame members 22 Frame socket ends 23 Frame plugends 24 Grid 30 Grid connectors 31

Mat 10 is preferably made of a water and gas permeable material such asa non-woven fabric. Materials Which as not permeable to water and gasesare lethal to all small benthic or that help sustain the health of abody of water. Preferably the non-woven fabric is of a polymericmaterial which is resistant to degradation and decay. Non-wovengeotextile fabric is preferable. It is a non-elastic material, but it issufficiently slightly stretchable, such that it can be made taut whenframe members 21 and 22 are located in pockets 13 and assembledtogether, with the sides of frame 20 bowing out slightly. Pockets 13 areformed by folding mat 10 over on itself and sewing or weldingoverlapping edge portion to the body of the mat.

Mat 10 is polygonal in configuration, e.g. rectangular as shown, withstraight sides (FIGS. 3 and 4). At its sides, mat 10 is formed intopockets 13, for receiving frame members 21 and 22 (FIGS. 1, 3 and 4).Mat 10 is cut away 12 its corners to facilitate insertion of framemembers 21 and 22 into pockets 13. The cut away portion is triangular,and preferably leaves a cut away corner sufficiently large that framemembers 21 and 22 can be conveniently joined at the cut away corner(FIG. 6). The term “cut away” as used herein refers to the fact that aportion of the coiner is missing. It does not mean literally that themat was actually “cut,” though that is one way to create the cut awaycorner. The corner of the mat could alternatively have been folded over,the mat could have been produced without the corner portion, etc.

Even though they are gas permeable, mats 10 also include gas reliefports 11, which facilitate release of gas which builds up beneathbenthic barrier 1 in use, faster than it can dissipate through thepermeable non-woven fabric of mat 10. These may be a small opening cutin mat 10, or a flap cut into mat 10, which covers the opening butallows gas build up to escape. The opening is about 1-3 inches indiameter. As shown in the drawings, the opening and its associated flapare semicircles, or “half-moons” with a 1.5 inch radius. However, otherconfigurations for the “port” could be used, e.g. “v” shaped, with orwithout flaps, rectangular, etc.

Frame 20 is formed of corner defining members 21 and straight framemembers 22. Corner members define the corners 21 c of frame 20, andinclude straight legs 21 a and 21 b extending away from corner 21 c. Leg21 a is longer while leg 21 b is relatively short. Straight framemembers 22 would typically be 4 to 6 feet long. The long legs 21 a ofcorner members 21 are preferably about 4 feet long, while short leg 21 bis about 1 foot long. The frame members are made of a material which isrigid, but the legs of which can be forced.

The ends of each frame member 21 and 22 serve either as a narrowed plug24 or a socket 23 for receiving plug 24 (FIGS. 5, 6, 7A and 7B). Plugend 24 is swaged to reduce its diameter so that it fits into the socketend of the next adjacent frame member (See especially FIGS. 7A and 7B).It both forms a tight joint, but also assists in allowing the longstraight legs of frame 20 to flex between corners.

The interior angle A encompassed by the corner portion 21 c of cornermember 21 is from about 1 to about 5 degrees greater than the interiorangle of the polygon defined by mat 10 (FIG. 5). Thus mat 10 isrectangular, such that its straight sides, extended to intersection,would encompass a 90 degree angle. However angle A in FIG. 5 is actually95 degrees.

To assemble frame 20 to mat 10, mat 10 is laid out flat on the groundwith the frame members 21 and 22 surrounding it, as shown in FIG. 4. Thestraight members 22 are inserted into pockets 13 first, one at a time.Pockets 13 are about 3-5 inches wide, which facilitates insertion andmanipulation of frame members into and within pockets 13. The end of thepreviously inserted frame member 22 is left exposed so the next framemember can be easily joined to it. The joints are preferably taped withduct tape.

The corner members 21 are joined last. The long leg 21 a of each cornermember is joined to its adjacent, straight leg at each corner. Becauseof the cut away corners 12 of mat 10, the joining of the socket end ofshort leg 21 b of each corner member to the plug end 24 of the free endof an adjacent straight member 22 is facilitated (See FIG. 6 and FIG.2). As noted above, the cut away portion is triangular, and preferablyleaves a cut away corner sufficiently large that frame members 21 and 22can be conveniently joined at the cut away corner (FIG. 6). Thus for acorner frame member with a short leg 21 b of about a foot in length, thehypotenuse of the cut away is about 25 inches such that the leas of thecut away triangle are about 18 inches. This leaves about 5-6 inches ofthe adjacent straight frame member 22 for a user to grasp, while joiningshort leg 21 b to straight, frame member 22 (FIG. 6). The joint betweenjoined plug 24 and socket 23 can be seen in FIG. 2, in the opening leftby the cut away corner portion of mat 10.

As shown in FIG. 6, the assembler has grasped the end of straight framemember 22 with his right hand, and the short leg 21 b of corner member21 with his left. He pulls his right hand towards his body while pushinghis left hand away, thus creating a separation between plug 24 andsocket 23. Plug 24 is then inserted into socket 23 to complete thejoining process, if the next adjacent corner or corners have alreadybeen joined, this joining will force the straight leg(s) 22 and/or 21 ato bow outwardly slightly, and will also force the insertion of plug 24into socket 23. Mat 10 is dimensioned such that this bowing of the sidemembers of frame 20 draws mat 10 taught, placing it in tension withinframe 10.

EXAMPLE

Consider a mat with the dimensions of 12-feet by 24-feet. By creatingcorners that are 95 degrees (instead of the 90-degree corners of astandard rectangle) the center of the frame bows out to 13-feet in thecenter. Stretching non-woven fabric, sewn 12-feet wide, over a framethat is 13-feet wide M the center, pulls the frame inward while pullingthe fabric taunt, much as a bow holds a bowstring taunt, by beingcambered outward, away from the bowstring.

This taut condition of mat 10 within frame 20 leads to unexpectedresults. Preventing photosynthesis through the use of a benthic mat isrelatively simple, and easily understood by those of ordinary skill inthe art. Releasing gases from, the lake bottom is not. As aquatic plantsdie, the resulting biomass is broken down and digested by bacteria.During the decomposition process, several gases are released includingoxygen (at first), nitrogen, carbon dioxide, and methane. Undigestedbiomass that sinks beneath the lake bottom soil must be digested byanaerobic bacteria, releasing ammonia, carbon dioxide and hydrogensulfide. Surface gases tend to dissipate quickly, within two months,provided there's enough oxygen and aerobic bacteria to digest the plantmatter. Deeper gases, like hydrogen sulfide, may have been created andtrapped for hundreds or thousands of years. All of these gases rise anddissipate, when allowed to.

However even in mats which are gas permeable, and even in those with gasrelief openings, these gases can bubble up so fast that they are trappedbeneath the benthic mat. As more gases accumulate, the entire benthicbarrier often lifts off the bottom, floating and losing its shape untilthe gases escape. Because assembled frame 10 holds the fabric mat 10taut, particularly in the center, gases migrate more efficiently to theedges of the mat, and to gas-release ports 11 if they are used. There isless “loose” fabric to form pockets where gases can collect.

Optionally, a grid 30 such as a geogrid can be joined to the assembledframe 20 and mat 10, through the use of zip ties or the like. This givesbenthic barrier 1 even greater stability on the lake floor, as it helpskeep the barrier from shifting. Grid 30 creates a firm surface on whichto walk over soft lake bottom soils. It acts as a giant “snowshoe” bydistributing weight over a larger area, and hinders “lateral” soilmovement.

This unique frame and fabric system allows for a relatively light, (60to 70 lbs. dry) yet very study and extremely effective product that iseasily installed, cleaned, moved and removed, it is stable in use,allowing gas bubbling up from below to flow readily to the edges and/orgas relief parts. Of course, it is understood that the forgoing is apreferred embodiment of the invention, and that various changes andalterations can be made without departing from the spirit and scope ofthe invention.

The invention claimed is:
 1. A method for preventing aquatic plantgrowth in the photic zone of a body of water, comprising: Positioning abenthic barrier on a portion of the bottom of said body of water,wherein said benthic barrier includes: a polygonal shaped mat havingstraight sides and corners between said sides; a frame having the samepolygonal shape as said mat, including corresponding sides and cornersbetween said sides, but in which said frame sides bow out between saidframe corners, said sides of said mat being secured to said sides ofsaid frame, whereby said outward bow of said frame sides cause said matto stretch and become taut.
 2. The method of claim 1 in which saidbenthic barrier frame comprises a plurality of straight leg members, anda number of corner members corresponding to the corners of the polygonalshape of said mat; said corner members encompassing interior angleswhich are from about 1 to about 5 degrees greater than the interiorangle of the polygon defined by said straight sides of said mat; saidframe member straight legs being rigid, but sufficiently flexible suchthat when assembled into said mat as an assembled frame, said straightlegs are forced to bow out between said corners, pulling said mat taut.3. The method of claim 2 in which said benthic barrier mat comprisespockets along each side into which said straight leg frame members areinserted as they are assembled into said mat.
 4. The method of claim 3in which said benthic barrier mat is cut away at its corners tofacilitate insertion of frame members into said pockets.
 5. The methodof claim 4 in which said benthic barrier is assembled by joining andinserting said straight members of said frame into said pockets of saidmats; then joining said corner members of said frame to said straightmembers by grasping the end of the adjacent straight frame member withone hand, and said corner member with the other, pulling said adjacentstraight frame member toward him and pushing said corner member away,creating a separation between the mating ends of said straight memberand forcing said straight member to bow out, and then coupling saidstraight member and said corner member together.